Why is it grainy in this part?
Artifatcs are caused by overlapping faces.

On the right side you have this situation (correct):



while on the left side (overlapped/dubled geometry):



You can clearly see the problem also in Solid view display mode.



As faces shares the same location, camera rays can't tell which one is visible from camera, so shows a bit of one and a bit of the other.

You have to create a gap between faces